Electrodes are sometimes used as the sole or auxiliary heating sources in glass-melting furnaces. The electrodes are typically positioned in the glass-melting furnaces by being extended through the walls of such furnaces. Typically, an "electrode block" of uniform composition is provided forming a portion of the wall and having a closed perimeter interior opening or passageway through which the electrode is extended into the furnace.
For many reasons, molybdenum is a generally accepted electrode material for use in glass-melting furnaces. One problem molybdenum experiences is that it oxidizes when heated above 500.degree. C. To overcome this problem, a common approach has been to provide the electrode in an assembly including a holder which supports the electrode and which circulates a cooling fluid such as water, inert gas or both about a portion of the electrode which is not covered by glass and therefore subject to oxidation.
Fluid cooling creates a wide thermal gradient between the surface of the fluid-cooled electrode assembly and the surrounding refractory electrode block, particularly at the innermost surface of the block forming the exposed inner surface or "hot face" of the furnace where the glass is melted and temperatures are greatest. The wide thermal gradient can cause cracking of the electrode block. The cracking involved is commonly referred to as "star cracking" an spreads radially outwardly from the electrode opening or passageway through the block. A crack on the hot face of the block encourages glass penetration which can lead to increased erosion/corrosion of the block.
In some instances, the consumption of electrode material is compensated for by inserting additional electrode material into the furnace. To do this with a conventional fluid-cooled electrode assembly, it has been necessary to stop the flow of cooling fluid. After insertion of the electrode material, cooling fluid is once again circulated through the assembly. This operation induces thermal shock stress in the electrode block surrounding the electrode assembly. Again, both type of thermal stresses (gradient induced and shock induced) tend to be relieved by cracking of the electrode block. There is always a concern with glass melting furnaces, which may be run continuously for months and even years, that increased erosion/corrosion due to cracking can necessitate the premature shutdown of the furnace and possible economic losses accompanying such shutdown.